For me it was ´~/Library/Application Support/iPhone Simulator´ in my home directory.
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BesiSep 20 '12 at 12:53

3

Not sure if this is happening for anyone else, but your Spotlight search may be surfacing an old version of the iOS simulator. Try opening the current Xcode's iOS simulator directly through terminal: open /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneSimulator.platform/De‌​veloper/Applications/iPhone\ Simulator.app/
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Neil SarkarOct 9 '12 at 18:00

@NeilSarkar: You should make your comment an answer
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Tim BütheJan 10 '13 at 19:52

In case you are using a dual graphics Mac (like a MacBook Pro) this also sometimes happens if vide system cannot switch to the discrete card. Resetting SMC fixes the issue: http://support.apple.com/kb/ht3964